To provide a battery separator wherein a captured amount of ammonia can be increased, and a self-discharging of the battery is suppressed when used in an alkaline secondary battery.

As for this battery separator, 2-acrylamide-2 methylpropane sulfonic acid and a monomer having carboxyl-group(s) is graft-polymerized with a substrate composed of polyolefinic fiber. Further, it is preferable to cross-link an acidic group of at least one kind of a monomer graft-polymerized at least on the surface via polyvalent metal(s). Further, by controlling the weight percentage of grafting material from 4 to 16%, the difference of theoretical ion exchange capacity, IEC (cal) calculated from the weight percentage of the grafting material and potassium ion exchange capacity, IEC (obs), actually measured can be made as 0.50 mmol/g or more, and the captured amount of the ammonia can be made 0.30 mmol/g or more.